1. Field of the Invention
This invention relates in general to fluid-cooled electrical apparatus, such as power transformers and in particular to an improved non-contaminating fluid circulating pump used in such apparatus.
2. Description of the Prior Art
It is well known that fluids, such as transformer oil, moving rapidly through small spaces create and dispell static electrical charges to the surrounding structures. When these charges accumulate on a grounded surface they rapidly and harmlessly are dispersed to ground. If, however, these charges are disposed of by the fluid to an insulated body, that body itself becomes charged. The magnitude of the charge buildup on such an insulated body is limited only by the degree of insulation of the body from ground. When the accumulated charge is sufficient on an insulated body to break down the insulation to ground, the body is discharged to such a point that the insulation, once again, holds the charge on the body. This discharge when moving through a fluid creates an arc which, with metallic materials and other solids, causes pitting of the solids at point of exit and entrance. Pitting is the result of material being eroded from the solid and, in the case of metals, a conducting debris is left suspended in the fluid and is free to move with the fluid, causing damage to bearings and journals.
In a transformer oil pump the rotating parts of the pump, such as the rotor assembly, are necessarily insulated from the stationary parts by natural gaps filled with oil. The moving parts provide ideal conditions for static charge buildup. The smallest of the gaps insulating the rotor is generally between the shaft journal and bearing, and it is across this surface that a discharge would most likely occur. The discharge effectively damages both the journal and the bearing by pitting and further transmits the debris to the oil. When a discharge occurs across the gap of bearing sensor rings and the shaft, even though this is outside of the journal-bearing area (protecting the bearing), debris is generated and subsequently suspended in the transformer oil. This is true for conducting as well as non-conducting bearings. The oil, being the parent insulating fluid moving through the transformer, moves the debris into the transformer, where it is deposited in critical spaces on the solid insulation, or allows it to remain in suspension in the fluid, and, in either case, move into areas of high electrical stress in the main transformer and subject the transformer to failure. By eliminating the formation of static charge buildup on transformer oil pump moving parts, the possibility of discharge to ground, damage to bearings and journals and formation of conducting debris can be eliminated. This can be accomplished by grounding these moving parts.
In a contamination-free pump, it is necessary to have an electrical circuit which guarantees that, should bearing wear occur to a predetermined amount, the power to the motor will be interrupted and the pump stopped. This circuit, to be a general circuit, must take into account all of the various types of power supplies to the pump. These can be grounded wye, grounded deltas, split deltas, ungrounded wye, ungrounded deltas, and others. Because of the variety of possible supplies and because of the difficulty of leads being carried into, through and around the motor parts in the motor housing of the pump, limiting the number of these leads is imperative. To allow the shut off circuit to function then with a minimal number of leads, it is desirable that one side of this "trip circuit" (or alarm circuit as the case may be) be grounded in the motor and ground be used as one complete side of the circuit.